For years, I have been going with a group of friends every week to eat at our favourite restaurant. The food was superb, but the volume of food was so huge, it was wasteful leaving it, so I ate till it hurt. To avoid this, I started taking half home each week, with the resultant foil boxes and cardboard lids and plastic bags going in the bin. After a few weeks deciding if food waste or packaging waste was worse, we all decided to take our own plastic boxes. The restaurants prefer not dealing with excessive food waste, I don’t overeat in the evening, I get a free lunch the next day, and there is no packaging waste.
You may think this is weird, but try it (just ask as you’re doing it). Restaurants would prefer you to enjoy your meal and come back than go out for food rarely.
As an alternative, if you know you are not going to eat it all and won’t take it home, ask for a smaller portion.